AWS D1.6 deals with welding of structural stainless steel, but I don't have a copy to see how much it deals with pipe applications.
Most commercial applications for Pipe do use the ASME code for qualification, and specs such as the B31.X series for constuction rules.
You might use one of the AWS Standard Welding Procedure Specifications to avoid having to perform the qualification and meet the AWS B2.1 standard. Here are some of the ones for stainless steel:
Austenitic Stainless Steel, Primarily Pipe Applications - Shielded Metal Arc Welding - B2.1-8-213-97
Gas Tungsten Arc Welding with Consumable Insert Followed by Shielded Metal Arc Welding of Austenitic Stainless Steel (M-8/P-8/S-8) - B2.1-8-216:1998
Austenitic Stainless Steel, Primarily Pipe Applications - Gas Tungsten Arc Welding - B2.1-8-212:2001
Marty